The present invention relates to an electronic control circuit for alternating current motors.
In known technology, the need to be able to switch an electric motor between full power condition and partial power condition in order to limit, for instance, its speed and/or the current absorbed and, consequently, the stress upon it, is frequently felt.
For example, some electric motors, in order to provide a high power-size ratio, use such windings that, when load-free and fully powered, they reach excessively high angular speeds, thus causing problems with regard to reliability and to safety standards. In an effort to get round these problems, control circuits have been produced which supply such motors with partial power whenever they are load-free, thus limiting the idling speed, and with full power whenever a load is exerted and therefore whenever the motor's full available power as required, without the risk of overspeed. In known technology, such circuits are usually produced with complex designs using expensive, special integrated control circuits or electromechanical systems using, for example, the centrifugal force produced by the rotation of the motor, which are unreliable in operation and often cumbersome in size.
Motors of the type just mentioned are, for instance, frequently used in portable electrical appliances for their favourable weight-power ratio. In such applications, it is obvious that the cost, compactness and safety of operation of the control circuit are features of primary importance and that known technology, as described, is barely satisfactory.
Another example of the need to switch between the two aforementioned conditions is the case when the current absorbed by an electric motor is to be limited to a maximum level, in order to prevent damage in the event of mechanical overloading. A special control circuit supplies full power to the motor as long as the amount of current absorbed does not exceed a predetermined level, above which the control circuit supplies the motor with partial power and, consequently, with reduced current thus safeguarding its integrity. In order to perform such an operation to avoid overloading, known technology relies on complex solutions which, for switching between the aforementioned power conditions, use expensive, special integrated circuits or bimetallic power cut-out devices, which are difficult to set and which operate in a discontinuous manner.
Limitation of current to avoid mechanical overload is also a feature which is desirable in portable electrical appliances given the extreme variability of the loads which such appliances may have to withstand when in use. In this case too, known technology proves to be unsatisfactory with regard to cost, compactness and safety of operation.
The general aim of the present invention is to prevent the above mentioned problems by providing a control circuit of simple construction, low cost and safe operation to supply alternating current motors, and particularly portable electrical appliances, with full power or partial power. A further aim of the present invention is to provide, with the said control circuit, a device for limiting the number of revolutions in no-load operations and, additionally, a device for limiting the current absorbed by AC motors.